The present invention relates to a transparent rear projection screen having a Fresnel-lens on its rear side for paralleling light arriving from behind, and which in the front surface of the screen has vertically extending grooves with side flanks, each of which adjoins a side part of an adjacent also vertically extending convex lens, said side flanks being totally reflecting to parallelled light arriving from behind in order to increase the deflection in the lateral direction of a part of the light arriving from behind by reflecting said light into the adjacent lens, whereby the angle which a side flank forms together with the adjacent lens side part is so small that total reflection from the inner surface of said lens side part of light which is totally reflected by means of the side flank in question is avoided, and wherein at least one further lens is arranged between the vertical lenses to which the side flanks adjoin.
A rear projection screen of the kind referred to above is disclosed in Danish Patent Application No. 878/83, filed Feb. 24, 1983 and claiming priority from Feb. 25, 1982 (Japanese Patent Application No. 29178/82) by Mitsubishi Rayon Co. Ltd. According to said application the further lens consists of a convex and as a part of a cylindrical surface shaped lens. This further lens is comparatively broad, seeing that it fills approximately one half of a pitch. By the term "pitch" as used in the present specification, the distance between the bottoms of two adjacent grooves is to be understood. Moreover, the angle which the totally reflecting side flanks form together with the normal to the screen is comparatively great, viz. approximately 20.degree.. The result is that the lenses to which the flanks adjoin become rather small and have a comparatively great inclination with respect to the normal to the screen. The result is that parallelled light which arrives from behind and impinges the flanks will be diverged and refracted strongly in the lateral direction when such light emits from the lenses to which the flanks adjoin. Moreover, a part of the light arriving from behind, viz. the light which impinges the parts of the flanks positioned adjacent the bottoms of the grooves will be totally reflected into the further lens so that this light is totally reflected from the inner surface of the further lens. This, in particular, will be the case when the rear projection screen is used for projecting colour television pictures which are produced by three projectors for television pictures, each of which projects its own picture towards the rear side of the screen, and wherein the pictures issued by each of the three projectors has its own colour, e.g. red, green and blue. Furthermore, if the further lens is to spread the parallelled light arriving from behind sufficiently in the lateral direction, the further lens must have a pronounced curvature, i.e. it must have a comparatively small radius of curvature. However, if this is the case the side parts of the further lens will form angles together with a normal to the screen so small that the side parts of the further lens will become totally reflecting to light arriving from behind, and accordingly, such light will not issue from the screen. Furthermore, a strongly curved further lens will result in that the light which issues from the further lens will hit the lenses to which the flanks adjoin whereby the path of rays, under the best conditions, will be randomly distributed. However, if the further lens is very flat, the spreading in the lateral direction will be correspondingly small, and a comparatively great part of the light arriving from behind will be issued in the direction of the normal to the screen which, however, has the effect that a person which views the screen in front thereof and opposite the centre of the screen will perceive the picture in question as being too light, whereas a person who views the screen under a greater or smaller lateral viewing angle will perceive the screen as too dark.